Transactions of the Korean Nuclear Society Virtual Spring Meeting July 9-10, 2020 Development of Computer Code for Analysis of Molten Corium and Concrete Interaction Sang Ho Kim, Sang Min Kim, Jae Hyun Ham, Hwan-Yeol Kim, Rae-Joon Park, Jaehoon Jung Accident Monitoring and Mitigation Research Team, KAERI, 989-111 Daedeok-daero, Yuseong-gu, Daejeon * Corresponding author: sangho@kaeri.re.kr 1. Introduction the results of these calculation tests for sub-functions, the COCCI is being developed with C++ code focused on wider usability and improved applicability. The integrity of the containment must be protected COCCI has the following general characteristics. and maintained in any kinds of accidents as it is the final First, COCCI is capable of modeling the physical physical barrier to prevent the release of the fission transient phenomena. A governing equation for each products. Proper actions must be taken so as to protect layer is set. The formation and growth of each crust the physical barriers. In order to establish the proper layer on the top, side, and bottom of the corium pool is mitigation strategy, the phenomena in accidents must be modeled. Options for transient concrete ablation and well identified and predicted. conduction heat transfer to side and bottom concrete After the reactor vessel breach in a severe accident of layers are included in COCCI. a conventional pressurized water-cooled reactor, the Second, a variety of analysis geometry coordinate corium falls down into the reactor cavity. The basemat systems are included in COCCI, which leads to a concrete ablation by the molten corium can threaten the decrease in the uncertainty from the scale ability and containment pressure boundary. Therefore, a cooling by coordinate system of an analysis code in the validation a specific facility or water injection system is needed for analysis for experimental tests. Otherwise, the the corium on the reactor cavity. In addition, the methodology for cavity shape change is simplified. accurate analysis for the molten corium-concrete Third, COCCI has wide usability. Options for models interaction (MCCI) and the cooling of the corium by recently developed and employed in CORQUENCH water is needed. code are provided to users with value recommendations In simulating the MCCI, there are various specific [4]. The models were reviewed for MCCI mitigation phenomena for consideration and also various models strategy in the recent research paper [5]. for each phenomenon. The uncertainties on the models have to be compared and identified. In addition, the 3. Calculation Flow of COCCI analytical uncertainties on scheme and system in a computer code have to decrease. Overall calculation flow of COCCI is illustrated in For this reason, the necessity of a computer code Fig. 1. When it is executed, all the input variables are capable of analyzing various coordinate system and read. All the initial and boundary conditions, parameters having various parameters for an uncertainty analysis and matrices are initialized for the time zero. has been raised. The objective of this paper is to During one time step, transfer rates for mass and develop the analysis system of a computer code named energy are solved. Then, the governing equations for Code Of Corium-Concrete Interaction (COCCI). The mass and energy are solved. There are various layers set general characteristics were also described. in the COCCI. Those include a center mixture liquid and each solid crust for top, side and bottom. The crust 2. General Description and Characteristics of formation and growth for each solid layer are calculated COCCI based on mass and energy equations. The concrete layers are divided into side and bottom. Each concrete COCCI is being developed to simulate the molten layer is ablated according to the option for the heat corium and concrete interaction in condition with or transfer in the concrete layer. The first option is the without coolant at the top. conventional ablation based on the decomposition Before the development of the COCCI, QuCCI temperature as the interface temperature. In the second (Quasi-stationary parametric simulation code for option, the conduction heat transfer in the concrete layer Corium-Concrete Interaction) was developed with is considered. In the third option, the transient ablation Python 3.6. As QuCCI has quasi-stationary energy term is modeled in the code. On the top of the corium layer, for concrete ablation and absorption with thermal the water and air layers are defined. The cooling equilibrium on melt, the modeling on mass transfer mechanism of water ingression or melt eruption can be through a stable crust was simplified. The sensitivity selected and included in the modeling as an user option. analysis was carried out for finding out the key variables After the calculation for mass, energy governing and in MCCI using QuCCI and MELCOR codes [1, 2]. It transfer equations, the convergence criteria are checked was also estimated that the effect of heat transfer models to calculate again with the reduced time step. If the and thermal conductivity on maximum crust thickness in calculation is completed for a time step, the concrete the equilibrium condition using QuCCI [3]. Based on
Transactions of the Korean Nuclear Society Virtual Spring Meeting July 9-10, 2020 geometry is updated and variable data are stored. After than that in WECHSL code simulation. It is caused by the calculation is complete for the total simulation time, the overestimation of the melt and concrete interface temperature, about 1500 ℃ at 300 min. It is estimated the data specified by an user are printed and the COCCI code terminates. that the deviation is caused by the constant thermal property and the absence of specific phenomenon models in the code system of COCCI. 5. Conclusions The basic analysis system and general characteristics for COCCI code were described in this paper. COCCI is capable of modeling the physical transient phenomena in MCCI condition. In addition, as a variety of analysis geometry coordinate systems are included in COCCI, the uncertainty from the scale ability and coordinate system will be reduced. The code analysis system was tested by modeling the CCI-2 experiment. Detailed Fig. 1. Overall Flow Diagram of COCCI specific models and material properties functions will be included in the COCCI hereafter. The COCCI will be 4. Validation Work coupled with an ex-vessel corium coolability analysis code (COCCA) for the comprehensive estimation with The development of COCCI is in progress. To check overall sequence and geometry in a reactor cavity. the calculation flow of the code and the variations of thermal values in layers, some representative experimental tests have been modeled and analyzed. In CCI-2 experiment, initial melt mass was 400 kg. Concrete type was Limestone/Common Sand (LCS). Density of the concrete was 2320 kg/m³. The initial mass of the melt was 400 kg. Initial melt temperature was 2153 K. Power supply operation was constant at 120 kW. The top surface was open to air for 5 hours. Water was injected after 5 hours. The water injection flow rate was 2 liter/sec. In addition, the simplified methodology for geometry update in COCCI has been tried. The initial melt geometry was cuboid whose volume was 0.5 (width) x 0.5 (thickness) x 0.25 (height) m³ in the atmospheric pressure. However, as two of four side-walls were insulated, the other two side-walls and a bottom-wall Fig. 2. Ablation Depth in CCI-2 test, and Code Simulations were modeled. The two side-walls and bottom-wall by COCCI and WECHSL code ablation were calculated in the axis-symmetric two- dimensional Cartesian geometry. The results of COCCI code are compared with those of WECHSL code developed in FZK, Germany [6] and also CCI-2 test. Fig. 2 shows the ablation depths for CCI-2 test and simulations. In the COCCI simulation, stable crusts were formed on the side of the corium layer for 36 minutes. The posttest examination for CCI- 2 showed the final axial ablated depth at the bottom wall was about 29 cm. Therefore, the COCCI simulation overestimated the axial ablation of concrete by 5 cm. In the case of the calculation results by the WECHSL code, the radial ablation was quite underestimated due to the different modeling for gases behavior near the side-wall and bottom-wall. Fig. 3 shows the corium temperature for CCI-2 test Fig. 3. Corium Temperature in CCI-2 test, and Code and code simulations. At 300 min from the initiation, Simulations by COCCI and WECHSL the corium bulk temperature was about 1,600 ℃ higher
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